Unveiling the molten pool evolution and defects under different processing parameters in pure tantalum fabricated via laser powder bed fusion

Tantalum (Ta) is an important biomedical material for the preparation of implants due to its excellent mechanical and biological properties. However, the presence of defects has a significant impact on the implantation effect. The stability and flow state of the molten pool, which is controlled by processing parameters, are of great significance for the formation of defect-free components fabricated by laser powder bed fusion (LPBF). To date, the relationship among processing parameters, molten pool evolution, and defects has not been established yet. In this paper, single-track and bulk samples of pure Ta were fabricated using LPBF technology, and a powder-resolved computational fluid dynamics (CFD) model of pure Ta was established to study heat transfer and fluid flow characteristics in the molten pool. The effects of processing parameter variations on the types and quantities of defects were systematically analyzed, and the formation mechanisms of defects were revealed by combining simulation results of molten pool evolution under different parameters. The results show that increasing the laser energy density would promote higher temperature and faster velocity fields within the molten pool. Under the elevated temperature field, the spreading speed was faster than the solidification speed of the molten Ta, and the melt of powders was more sufficient, resulting in fewer keyholes after solidification. Under the rapid velocity field, the Marangoni convection was intense, which would lead to difficulty for bubbles to escape and an increase in gas pores. The coordination of temperature and velocity fields in the molten pool determined the types and quantities of defects in LPBF samples. Unmelted Ta powders acted as barriers to prevent the epitaxial growth of grains, depending on the melting state. This article presents a profound analysis of processing parameters, defects, molten pools, and microstructure changes, and is expected to provide scientific and theoretical support for the exploration of LPBF processing parameters and the causes of changes in the microstructure and properties of LPBF samples.